Adsorption of pulmonary surfactant protein D to phospholipid monolayers at the air-water interface

The intrinsic surface activity of recombinant rat surfactant-associated protein D (SP-D) expressed in CHO-K1 cells has been determined from measurements of surface tension of its aqueous solutions. The interactions of recombinant SP-D with monolayers of phosphatidylcholine (PC), phosphatidylglycerol (PG), and phosphatidylinositol (PI) spread at the air-water interface have been characterized. Injection of SP-D beneath preformed lipid monolayers at surface pressures less than 30 mN/m produced an increase in surface pressure, consistent with SP-D penetrating the lipid films. The adsorption of SP-D to the lipid monolayers did not display significant head group dependency, suggesting that the changes in surface pressure produced by the protein were likely due primarily to hydrophobic interactions with the lipid layers. In the presence of calcium ions in the subphase, SP-D displayed lower surface activity by itself and a reduced ability to generate surface pressure changes during adsorption to lipid monolayers compared to these properties of the protein in the absence of 2 mM Ca2+. Circular dichroism measurements on SP-D solutions with or without Ca2+ suggested that the cations altered the conformation of the protein and this possibly led to the calcium dependency of the surface activity of the protein in the presence or absence of lipid monolayers. Compressional isotherms of surface pressure versus area for SP-D/(DPPC-PI) and SP-D/(DPPC-PG) films formed by adsorption of the protein to preformed lipid monolayers were consistent with incorporation of some or all of the SP-D molecules into the lipid layers. The isotherms obtained on compression of the SP-D/lipid films to a maximum surface pressure of about 70 mN/m were consistent with the interpretation that any SP-D which was incorporated by adsorption was apparently not squeezed out, nor was lipid removed by the protein. The work suggests that when soluble recombinant SP-D is allowed to interact with phospholipid monolayers under the selected conditions of this experiment, it does so to a limited extent driven by primarily hydrophobic forces, and apparently without a high selectivity for phospholipid head groups.